Extra cavity frequency-converted pulsed solid state lasers are used extensively for material processing applications such as machining, drilling, and marking. Most commercially available, pulsed, solid-state lasers are Q-switched pulsed lasers. Q-switched pulsed lasers include a laser-resonator having a solid-state gain-element and selectively variable-loss device located therein. The laser resonator is terminated at one end thereof by a mirror that is maximally reflecting at a fundamental wavelength of the gain-element, and terminated at an opposite end thereof by a mirror that is partially reflecting and partially transmitting at the fundamental wavelength. Such a laser is usually operated by continuously optically pumping the gain element while periodically varying (switching) the loss caused by the variable loss device (Q-switch) between a value that will prevent lasing in the resonator and a value that will allow lasing in the resonator. While lasing is allowed in the resonator, laser radiation is delivered from the partially transmitting mirror as a laser pulse.
The pulse repetition frequency (PRF) of a Q-switched solid-state laser is determined by the frequency at which the Q-switch is switched. The pulse duration is determined for any particular gain-medium by factors including the length of the resonator, the transmission of the partially-transmitting mirror, losses in the Q-switch in a “lasing-allowed” condition, the optical pump power, and the PRF. A pulse repetition rate and pulse duration that are optimum for an operation on any one material will usually not be optimum for another operation or another material. Accordingly, an “ideal” pulsed laser would have independently variable PRF and pulse-duration to allow an optimum combination to be selected for most operations on most materials.
One type of laser system in which the PRF can be varied without a variation in pulse duration is a fiber-based MOPA in which seed pulses are generated by a master oscillator in the form of a modulated, edge-emitting semiconductor laser (diode-laser) and amplification is provided by a fiber-amplifier. A fiber-amplifier has relatively high gain, for example between about 13 decibels (dB) and 30 dB. This, combined with a low saturation power, allows a variety of low-power diode-laser seed sources to be used. Such a fiber MOPA can be operated at PRFs from less than 100 kilohertz (kHz) to 5 Megahertz (MHz) or greater, with pulse durations selected between about 0.1 nanosecond (ns) and about 1 microsecond (μs).
A significant problem in fiber-amplifiers is created by nonlinear effects in fibers which limit peak power and affect the spectral characteristics of the optical pulses. For harmonic generation at nanosecond pulses, spectrally-narrow light with a bandwidth between about 0.5 nanometers (nm) and 1.0 nm is required. Stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and spectral broadening of nanosecond pulses due to four-wave mixing (FWM) in fibers significantly narrow the available space of optical parameters acceptable for frequency conversion.
Stimulated Raman scattering (SRS) limits the peak power in typical all-fiber systems with core diameters below 30 μm (so called LMA fibers) although larger diameters of up to 100 μm are also possible using specially designed fibers (so called photonic-crystal fibers and leaky-mode fibers). SRS is the only power-limiting factor for broadband, for example greater than 1.0 nm bandwidth, IR nanosecond pulses. Attempts to narrow the spectral bandwidth using a single-frequency seed source encountered a build-up of stimulated Brillouin scattering (SBS) resulting in optical damage of fiber components.
It is known that for long optical pulses, for example pulses having a duration greater than 20 ns with a bandwidth much broader than for SBS, for example between about 0.20 picometers (pm) and 0.25 pm (between about 40 and 50 MHz), the threshold power for SBS grows proportionally to the signal spectral width. This is why a common approach for SBS suppression in long pulses is to broaden the pulse bandwidth to be much larger than the SBS bandwidth of about 0.2 pm while keeping the pulse bandwidth below about 1 nm that is appropriate for frequency conversion. However, spectral broadening of nanosecond pulses due to four-wave mixing in fibers transfer the energy from the narrow spectral peak to the spectral wings at pulse peak powers above 100 W (FIG. 1). This effect reduces frequency-conversion efficiency in all-fiber systems compared to solid-state lasers.
Another way to reduce SBS is to use pulses shorter than the SBS build-up time in fibers, which is typically close to 20 ns. For pulses having a duration less than 20 ns, SBS occurs in a transient regime with a smaller gain-factor.
There are two common approaches to generate pulses with variable length and pulse repetition rate. The first approach uses a directly modulated=diode-laser as a seed source. Such an approach is, in general, less expensive, and provides high peak power, for example greater than 1 Watt (W) from the seed laser. A major disadvantage of this approach is that to provide short pulses having a duration of less than 10 ns a short cavity length, for example less than about 10 millimeters (mm), is required. This, in turn, results in a single-frequency or “few-frequency mode” operation that favors SBS and limits the peak power in fiber-amplifiers. For this reason, this approach is limited to short pulses (duration less than about 10 ns), where SBS exhibits a reduced gain. Using broadband reflectors in the cavity and generating more modes helps to reduce SBS but immediately results in stronger broadening of the spectrum due to FWM, making frequency-conversion inefficient.
The second approach uses a continuous wave (CW) optical source or pulsed optical source with long pulses, modulated by an external modulator. In such a system, a seed source could be a diode-laser, a solid-state laser, a fiber laser, or a source generating amplified spontaneous emission (ASE source), such as a superluminescent LED, while a typical modulator is an electro-optical crystal in the waveguide Mach-Zehnder configuration or a semiconductor optical amplifier.
On one hand, such an approach provides less peak power (typically less than 100 mW) after modulation compared with that available from a directly modulated diode. On the other hand this approach allows generation of pulses of any length and repetition rate with a spectrum determined by an appropriately designed seed laser. For example, a diode seed laser may have a low noise operation when a fiber Bragg grating (FBG) written in a fiber placed in 1-2 m from a diode-laser chip provides an output coupler for the diode-laser cavity. With a typical FBG bandwidth of between about 0.01 nm and 1.0 nm, such a diode-laser will have a much broader spectrum than the SBS bandwidth, which helps to suppress SBS. However, due to mode-beating such a source exhibits pulse-to-pulse fluctuations, especially when operating with short pulses of less than 10 ns duration. The second approach is therefore limited to pulse durations of greater than about 10 ns.
For many applications it would be desirable to have a fiber MOPA with a frequency conversion stages operating at any pulse duration between about 0.1 ns and 1 microsecond.